No. 6 - Secrets of the North
Unlike Beneath Cursed Sands, Secrets of the North features a final boss which is replayable after the quest, albeit with increased stats.
The battle against the Phantom Muspah, known simply as Strange Creature during the Master quest itself, can be challenging to learn, given how many mechanics there are to prepare for.
This boss alone, from its attack style changes to the prayer shield to the barrage of orbs to the various spike attacks to the shockwave, could easily have this quest in Grandmaster territory consideration.
And don’t forget the fact that there are two bosses to deal with beforehand. Evelot can disable protection prayers, and the Assassin can only be damaged by luring him into his own smoke bombs, all while avoiding his green poison vials.
There isn’t a whole lot to this quest aside from the combat encounters, but they are no joke. Preparing for each fight before embarking on the journey is highly recommended, and that alone could turn what is a relatively short quest into a long one.